The present invention relates to a hydraulic shock absorber suitable for use to damp or buffer vibrations of a vehicle, for example, an automobile.
In general, a hydraulic shock absorber has an outer cylinder, one end of which is closed, and which has an annular cover member provided at another end thereof. An inner cylinder is provided in the outer cylinder to form an annular reservoir chamber between itself and the outer cylinder. A piston rod is inserted at one end thereof into the inner cylinder. Another end of the piston rod extends through the cover member and projects to an exterior of the outer cylinder. A cylindrical rod guide is provided at another end of the inner cylinder to slidably guide the piston rod. A rod seal is provided on an inner periphery of the cover member in sliding contact with an outer periphery of the piston rod to seal between the piston rod and the cover member.
With the hydraulic shock absorber according to related art as described above, the one end of the outer cylinder is secured to an axle-side member of an automobile, for example, and the another end of the piston rod is secured to a body side member of the automobile. When vibrations occur during running of the automobile or on other occasions, the piston rod extends and retracts to cause hydraulic fluid to flow between a rod-side hydraulic fluid chamber and a bottom-side hydraulic fluid chamber that are formed in the inner cylinder, thereby generating damping force through a damping force generating mechanism including disk valves so as to damp the vibrations of the automobile.
In the related art, when the piston rod retracts or extends, a volumetric capacity of the inner cylinder changes by an amount corresponding to an amount by which the piston rod enters or withdraws from the inner cylinder. At this time, therefore, hydraulic fluid in the inner cylinder is allowed to flow out into the reservoir chamber, or hydraulic fluid in the reservoir chamber is allowed to flow into the inner cylinder. In addition, a gas sealed in the reservoir chamber is compressed or expanded by action of hydraulic fluid flowing into or out of the reservoir chamber. In this way, such a volumetric change in the inner cylinder is absorbed.
Further, in the related art, a hydraulic fluid reservoir chamber is provided between the rod seal and the rod guide. In addition, a lip seal is integrally provided on an outer periphery of the rod seal. The lip seal is located between the reservoir chamber and the hydraulic fluid reservoir chamber to serve as a check valve. The lip seal separably abuts the rod guide to allow hydraulic fluid in the hydraulic fluid reservoir chamber to flow toward the reservoir chamber, and to block flow of hydraulic fluid in a reverse direction.
Accordingly, when the rod-side hydraulic fluid chamber is brought into a high-pressure state during extension of the piston rod, hydraulic fluid in the hydraulic fluid chamber leaks into the hydraulic fluid reservoir chamber through a gap between the piston rod and the rod guide. As a result, an excess of hydraulic fluid, due to leakage thereof into the hydraulic fluid reservoir chamber, causes the lip seal to open and flows therethrough to return to the reservoir chamber. Thus, pressure of hydraulic fluid in the hydraulic fluid reservoir chamber is kept from increasing excessively to prevent an excess load from acting on the lip seal.
During assembly of a hydraulic shock absorber, air enters the inner cylinder undesirably. If the hydraulic shock absorber is used in this state, damping force characteristics obtained with the disk valves degrade. Therefore, in advance of shipment of products or on other occasions, an operator performs an air removing operation to discharge air, in the inner cylinder, into the reservoir chamber, thereby preventing degradation of quality of the products.
More specifically, during the air removing operation, the piston rod is caused to extend and retract a plurality of times, thereby releasing air, in the inner cylinder, into the hydraulic fluid reservoir chamber through the gap between the piston rod and the rod guide. The released air opens the lip seal and is discharged into the reservoir chamber through the lip seal.
Incidentally, with the above-described related art, if interference of the lip seal abutting the valve seat surface of the rod guide is reduced so as to set a valve opening pressure of the lip seal at a lower level, hydraulic fluid and air leaking into the hydraulic fluid reservoir chamber can be returned to the reservoir chamber smoothly.
However, if a valve opening pressure is reduced simply by reducing a thickness of the lip seal, some problems arise. That is, the rod seal itself, or a member provided around the rod seal (e.g. the rod guide), needs to be produced with some dimensional tolerances. Therefore, if the dimensional tolerances of these members are large, when the hydraulic shock absorber is assembled, the lip seal, which is integral with the rod seal, may strongly abut the rod guide at an entire surface thereof, causing interference of the lip seal to become larger than is needed.
In such a case, as interference of the lip seal increases, a valve opening pressure increases. Consequently, removability of air from the inner cylinder degrades. In addition, variations in interference of the lip seal due to dimensional tolerances of the rod seal and the like make it difficult to keep a valve opening pressure of the lip seal small and uniform among products.
In the related art, further, the rod seal having the lip seal is integrally secured to the cover member, and the cover member is secured to the another end of the outer cylinder by bending the another end of the outer cylinder against the cover member. Therefore, interference of the lip seal varies also with bending pressure applied to the cover member when secured to the another end of the outer cylinder. Accordingly, it is difficult to make a valve opening pressure of the lip seal uniform among products.
The present invention was made in view of the above-described problems with the related art, and an object of the present invention is to provide a hydraulic shock absorber designed so that a valve opening pressure of a lip seal can be kept small irrespective of size of the interference of the lip seal, and it is possible to minimize variations in valve opening pressure of the lip seal among products and to improve air removability.
To solve the above-described problem, the present invention provides a hydraulic shock absorber including an outer cylinder, one end of which is closed, and which has an annular cover member provided at another end thereof. An inner cylinder is provided in the outer cylinder to form an annular reservoir chamber between itself and the outer cylinder. A piston rod is inserted at one end thereof into the inner cylinder. Another end of the piston rod extends through the cover member and projects to an exterior of the outer cylinder. A cylindrical rod guide is provided at one end of the inner cylinder to slidably guide the piston rod. A rod seal is provided at an inner periphery of the cover member in sliding contact with an outer periphery of the piston rod to seal between the piston rod and the cover member. A hydraulic fluid reservoir chamber is formed between the rod seal and the rod guide to accommodate hydraulic fluid leaking out of the inner cylinder through the rod guide. A check valve is provided between the reservoir chamber and the hydraulic fluid reservoir chamber. The check valve is separably seated on the rod guide to allow hydraulic fluid in the hydraulic fluid reservoir chamber to flow toward the reservoir chamber, and to block flow of hydraulic fluid in a reverse direction. The check valve has a hydraulic fluid groove for guiding hydraulic fluid from the hydraulic fluid reservoir chamber to an outer peripheral portion of the check valve. Preferably, the check valve is formed from a lip seal having an annular lip portion integrally formed on an outer periphery of the rod seal and extending radially outwardly. The annular lip portion is separably seated on the rod guide. Preferably the annular lip portion of the lip seal is formed with the above-described hydraulic fluid groove extending from a radially inner side toward a radially outer side along one side surface of the lip seal that faces the hydraulic fluid reservoir chamber.
In one embodiment of the present invention, the annular lip portion of the lip seal serving as the check valve is provided with the hydraulic fluid groove extending from the radially inner side toward the radially outer side along the one side surface of the lip seal that faces the hydraulic fluid reservoir chamber, so that hydraulic fluid in the hydraulic fluid reservoir chamber can be guided to an exterior of the hydraulic fluid groove provided on the annular lip portion of the lip seal. Accordingly, even when there is an increase in interference of the annular lip portion abutting the rod guide when the lip seal is installed, passage of the hydraulic fluid groove is ensured. Therefore, only an outer portion of the annular lip portion opens, and a valve opening pressure of the lip seal can be set at a low level.